Method of making an improved refractory boat for metal vaporization

ABSTRACT

A self-resistance heated boat for metal vaporization is manufactured by hot pressing suitable refractory powders, including at least one that is electrically conductive, to a suitably dense ingot. An elongated boat is then machined from the ingot so that the longitudinal dimension of the boat is substantially perpendicular to the direction of pressing.

United States Patent 1191 Passmore et al.

1451 Apr. 16, 1974 1 1 METHOD OF MAKING AN IMPROVED REFRACTORY BOAT FOR METAL VAPORIZATION [75] Inventors: Edmund M. Passmore, Wilmington,

Mass; Wilfrid G. Matheson, San Diego, Calif.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

22 Filed: July 2,1973

21 Appl. No.: 375,501

Related US. Application Data [63] Continuation of Ser. No. 236,487, March 21, 1972,

abandoned.

[-52] U.S.- CI......-;...,...- "129/611, 219/275 264/67 [51] Int. Cl. H05b 3/00 [58] Field of Search 29/610, 611, 527.1;

[56] I References Cited UNITED STATES PATENTS 2,962,538 11/1960 Alexander 13/25 Mundorl', Jr. 117/107 Primary Examiner-Charles W. Lanham Assistant ExaminerVictor A. DiPalma Attorney, Agent, or FirmJames Theodosopoulos [5 7] I ABSTRACT A self-resistance heat ed boat for metal vaporization is manufactured by hot pressing suitable refractory powders, including at least one that is electrically conductive, to a suitably dense ingot. An elongated boat is then machined from the ingot so that the longitudinal dimension of the boat is substantially perpendicular to the direction of pressing.

3 Claims, No Drawings METHOD OF MAKING AN IMPROVED REFRACTORY BOAT FOR METAL VAPORIZATION CROSS REFERENCE TO RELATED APPLICATION This is a continuation in part of Ser. No. 236,487, filed Mar. 21, 1972 now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to self-resistance heated refractory boats used as evaporation sources in vacuum metallizing apparatus. It particularly relates to a method of making an improved boat.

2. Description of the Prior Art Evaporation sources for vacuum metallizing processes, such as for the continuous deposition of aluminum on, for example, rolls of plastic film, generally comprise a refractory boat to contain the metal to be evaporated or to which the metal can be fed, for example, from a spool of wire. Such boats-can be either heated by a separate heater or can be self-resistance heated. Examples of the former are shown in US. Pat. Nos. 2,772,318, 3,063,865, 3,373,260, 3,514,575, 3,515,852, 3,636,303, 3,636,304 and 3,636,305. In these patents, the boat and the heater comprise separate bodies, the boat comprising material that is resistant to the metal to be evaporated and the heater comprising metal suitable for electric resistance heating, induction heating and the like. Examples of the latter are shown in US. Pat. Nos. 2,730,986, 2,866,724, 2,903,544, 2,962,538, 2,984,807, 3,181,968, 3,216,710 and 3,544,486. The boats disclosed in these patents are electrically conductive and are heated by passing electric current therethrough.

In some applications, self-resistance heated boats are preferable because they can evaporate metal, for example, aluminum, at a faster rate than can separately heated boats for the electrical capacity of a particular evaporation apparatus.

One of the problems of prior art self-resistance heated boats is the short life thereof, especially at the high evaporation rates employed during some vacuum coating processes. It is an object of this invention to manufacture a self-resistance heating boat the life of which is greatly increased over that of prior art boats.

SUMMARY OF THE INVENTION In the manufacture of a self-resistance heating evaporating boat in accordance with this invention, refrac' tory powders including at least one that is electrically conductive are thoroughly mixed together to yield a homogeneous blend. The blend is then placed in a mold and hot pressed between two parallel jaws to yield a machinable ingot. The transverse dimension-of the ingot, that is to say, the dimension thereof that is perpen-. dicular to the direction of pressing, must at least equal the length of the boats to be made from the ingot. For efficient utilization of material, said dimension should substantially exceed the boat length if cylindrical molds are used.

Boats are then machined from the ingot so that the long dimension of each boat corresponds to'the transverse dimension of the ingot, that is to say, is substantially perpendicular to the direction of pressing. Also, a suitable cavity may be machined into the upper surface of each boat for the purpose of containing metal to be evaporated. Examples of boats having such cavities are shown in US. Pat. Nos. 2,962,538, 2,984,807, 3,181,968 and 3,216,710.

Examples of refractory powders that may be used in manufacturing boats according to this invention are aluminum nitride and boron nitride; the electrically conductive refractory powder is titanium boride.

DESCRIPTION OF THE PREFERRED EMBODIMENT In one embodiment of this invention, a composition as described in US. Pat. No. 3,544,486, the disclosure of which is incorporated herein by reference, was prepared. The composition consisted of a blend of the following refractory powders: 45.9 percent titanium boride 30.4 percent boron nitride and 23.7 percent aluminum nitride.

The powder blend was placed in a cylindrical heatable graphite mold having graphite punches at each end thereof and was then hot pressed for 2 hours at 1,800 C at 5,000 p.s.i. After removal from the mold, the pressed body consisted of an ingot 4.14 inches in diameter by 4.75 inches long, having a density of 2.72 grams/cc, 84 percent of the theoretical density for the composition employed.

Two elongated evaporation boats, each 3 inches long threefourth inch wide by three-eighth inch thick and having a cavity 2 54; inches long by nine-sixteenth inch wide by one-fourth inch deep, were then machined therefrom. One boat (A) was machined in the usual manner employed in the art, that is to say, with the long dimension of the boat parallel to the direction ofpressing. The other boat (B) was machined so that the long dimension of the boat was perpendicular to the direction of pressing.

The boats were then tested by self-resistance heating to the evaporating temperature range of aluminum in a vacuum chamber and feeding 60 mil aluminum wire into the cavity at a rate of one gram per minute and evaporating same therefrom. i

The boats were tested as follows: a boat would be mounted in the vacuum evaporation apparatus, raised to evaporating temperature, and aluminum wire would be fed into the boat cavity. The temperature of the boat and rate of aluminum feed would be controlled so that aluminum was evaporated at a rate of 1 gram per minute for 50 minutes; then 2 grams per minute for 25 minutes; then 3 grams per minute for 17 minutes; then 4 grams per minute for 12% minutes then 5 grams per minute for 10 minutes; and finally 6 grams per minute for 8 minutes. The total evaporation would be about 300 grams of aluminumin about 125 minutes. Periodic observation was conducted so that a test couldv be discontinued when a boat cracked.

After grams of aluminum had been evaporated from Boat A, the boat had developed cracks transverse to the longitudinal dimension of the boat and was no longer usable.

However, 250 grams of aluminum were evaporated from Boat 18 before cracks developed therein of about the same magnitude as in Boat A, after the 100 grams evaporation. But, in addition, the cracks in Boat B were longitudinal in direction, not transverse as in the case of Boat A, and did not prevent continued use of the boat for evaporating aluminum.

Transverse cracks can result in catastrophic failure of the boat by physically rupturing it into two or more pieces accompanied by loss of electrical continuity. However, longitudinal cracks do not result in catastrophic failure even if the cracks extend the whole length of the boat since there is no loss of electrical continuity, and a boat having such cracks can continue to efficiently evaporate aluminum. The advantage of boats made in accordance with this invention is significant in the vacuum coating of rolls of paper or plastic, where it is especially desirable 'to prevent catastrophic failures of the evaporating boats during the coating of a single large roll. The coating of such a roll can require 8 hours, or even more.

A convenient method of manufacturing boats in accordance with this invention is to hot press a blend of refractory powders, selected from those previously mentioned, into an ingot, the axis of the ingot coinciding with the direction of pressing. The ingot is then cut into slabs, using, for example, a diamond saw, the thickness of each slab equalling or slightly exceeding the desired thickness of a finished boat. Each slab is then cut into rectangular blocks, the length and width of each of which equal or slightly exceed the desired length and width of a finished boat. It can be seen then, that the longitudinal dimension of each block is perpendicular to the direction of pressing of the blend of refractory powders.

Each block can then be dressed or finished to the final boat dimensions by, for example, grinding or milling and a cavity can be ground or milled into the upper surface of the boat.

In another example, a cylindrical ingot made of 33 percent TiB 43.5 percent BN and 23.5 percent AlN was hot pressed to a diameter of 4.14 inches and a height of 3.95 inches. Two boats made as per the prior art, that is, with their longitudinal dimension parallel to the axial direction of the ingot, were machined from this ingot. Two boats made in accordance with this invention, that is with their longitudinal dimensions perpendicular to the axial direction of the ingot, were also machined from the same ingot. All four boats were tested in accordance with the procedure mentioned above. Of the two prior art boats, one had much cracking after only 50 grams of aluminum had been evaporated therefrom and one had slight cracking after grams of aluminum had been evaporated therefrom. Of the two boats in accordance with the invention, one had no cracking after 100 grams of aluminum had been evaporated therefrom, and one had slight cracking after 250 grams had been evaporated therefrom.

The range of compositions employed in evaluating three-component boats in accordance with this invention was as follows: titanium boride about 20 to 50 percent; boron nitride about 10 to 65 percent; and the balance aluminum nitride. In two-component boats, the ranges were 20 to 80 percent titanium boride and 20 to 80 percent boron nitride. In all cases covering these ranges of compositions, boats made in accordance with this invention proved superior, both in life and resis tance to cracking, to boats of the same composition but made as per the prior art.

In commercial operation in the coating of rolls of paper and plastic with aluminum, boats made in accordance with this invention have proven to have a life about 60 to percent greater than prior art boats.

We claim: 1

1. In the method of manufacturing a self-resistance heated evaporating boat made by preparing a blend of titanium boride and boron nitride or a blend of titanium boride, boron nitride and aluminum nitride, charging the blend into a heatable mold, hot pressing the blend into a cylindrical machinable ingot, the direction of pressing being parallel to the axis of the cylindrical ingot, and removing the ingot from the mold, the improvement which comprises: machining an elongated boat from the ingot so that the longitudinal dimension of the boat is in a direction that is perpendicular to the axial direction of the ingot.

2. The method of claim 1 wherein the compositional ranges of titanium boride and boron nitride in the blend of titanium boride and boron nitride are between about 20 and 80 percent.

3. The method of claim 1 wherein the compositional ranges of the blend of titanium boride, boron nitride and aluminum nitride are as follows: titanium boride about 20 to 50 percent; boron nitride about 10 to 65 percent; balance is a aluminum nitride.

* k FR UNITED STATES PATENT OFFICE CERTIFYC AT E OF CORRECTION In e Edmund M. Pas-smote: et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Item [65] should read Continuation-in-par't of ser. No. 256,487, March 21, 1972, abandoned.

Signed and sealed this 1st day of October 197A.

(SEAL) Attest:

MoCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-DC 60376-P69 u s sovummzm PRINTING OFFICE: 930

FORM PO-1050 (10-69) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 1nventor(s) Edmund M. Passmor'el et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

i Item [65] should read Continua-tion-in-part of Ser. No. 256,L+87, March 21, 1972, abandoned.

Signed and sealed this 1st day of October 197 (SEAL) Attest: v

MCCOY M. GIBSON JR. C. MARSHALL DANN Arresting Officer Commissioner of Patents QRM O-1050 (10-69) USCOMM-DC 50376-P69 U 5. GOVERNMENT PRINTING OFFICE: 8 69 93 o 

2. The method of claim 1 wherein the compositional ranges of titanium boride and boron nitride in the blend of titanium boride and boron nitride are between about 20 and 80 percent.
 3. The method of claim 1 wherein the compositional ranges of the blend of titanium boride, boron nitride and aluminum nitride are as follows: titanium boride about 20 to 50 percent; boron nitride about 10 to 65 percent; balance is a aluminum nitride. 